That is a 700 series MTD mower made for Craftsman.
Go here
http://support.mtdproducts.com/app/answers/detail/a_id/10584/related/1/session/L2F2LzEvdGltZS8xNDk4NDA1MzU0L3NpZC9vdUFYcSpsbg%3D%3D and scroll down to the ride on manuals and download it.
As previously noted there is a double pulley just under the battery box.
Take both belts off and check that the middle section of the pulley can move up and down.
They occasionally get stuck.
Also unless the belts specifically say MTD replacement, aftermarket belts will not work properly on this mower.
I have had mixed luck with the Stens belts but the Rotary ones seem to work reasonably well, other wise use original MTD belts, they are different to standard section V belts.
The manual explains how to change the belts and check the varidrive pulley.
Also check the very large pulley on top of the gearbox, they do work loose.
Do not do it up with a rattle gun as the shaft is hollow and breaks really easy.
Found that out the hard way.